In the oil refining and petrochemical industries, heat transfer units are commonly used to raise the temperature of a gas, liquid, or other multi-phase fluid to higher temperatures as required for a variety of downstream processing operations. Depending on the process and the heat transfer needs, the heat transfer unit can be a heat exchanger, boiler, fired heater, or other accepted heat transfer systems. In such systems, it is common for an input fluid of a lower temperature to be heated to a higher temperature by passing the fluid through a tube, between plates, or through other conduits where an external heat source applies a heat flux (which often is expressed as the rate of heat transfer per unit area) across the conduit in order to raise the temperature of the fluid flowing therein. In such systems, the heat transfer occurs through conductive heating, convective heating, radiant heating, or a combination of conductive, convective, and radiant heating, as well as other heat transfer mechanisms. Convective heat transfer generally involves a thermal energy exchange between a surface and a moving fluid. Conductive heat transfer typically involves the transfer of thermal energy through a solid or liquid from a region of high temperature to a region of low temperature. Radiant heat transfer is the transfer of thermal energy by radiation from a surface or other source.
In conventional heat exchangers or boilers, for example, the heating medium is provided at a relatively consistent flow along the length of conduit in the heat exchanger so that a relatively consistent heat transfer is obtained along the entire length of conduit. Such systems typically provide a relatively consistent heat flux along the length of the conduit, and fluctuations in the heat flux are preferably minimized. In other cases, such as in a fired heater for example, the tubing or conduit of the furnace extends through a heater box containing one or more burners therein to provide a radiant heat source to increase the temperature of the fluid flowing through the tubing. In these radiant heating systems, the radiant heat flux can vary substantially along the length of heater tubing so that a relatively inconsistent heat transfer occurs along the length of tubing. A peak radiant heat flux generally occurs along the tubing closest to the radiant heat source and the heat flux decreases the farther the tubing is from the heat source.
Often a fired heater is required to heat the gas, liquid, or other multi-phase fluid to temperatures of about 537° C. (1000° F.) or greater, which requires a relatively high heat flux to be applied to the heater coil. In such instances, due to the high heat flux, particularly at those portions closest the heat source, the coils are commonly fabricated from materials capable of withstanding high temperatures, which are often exotic metal alloys such as chromium-molybdenum steels and certain stainless steels. While these materials have the capability to withstand continuous high temperatures, they typically are expensive and have low thermal conductivity that imparts additional challenges into the fired heater design.
In fired heater designs, it is generally desirable to minimize the pressure drop through the process heater. In some instances, the allowable pressure drop is so low that it is necessary to design process heaters with a low mass velocity in order to minimize the pressure drop through the heater tubing. The low mass velocity, however, can result in a reduced convective heat transfer coefficient, which may cause a large temperature rise across a relatively stagnant boundary film at the inside diameter of the tube. A large temperature rise across this boundary film tends to cause an increase in the tube wall temperature (TWT), which can result in a TWT that exceeds design limits of the tube metallurgy. Increasing the tube surface area, typically by increasing the tube length to reduce TWT, results in a greater pressure drop through the heater, which may require a further reduction of the mass velocity. Decreasing the mass velocity rate further results in a corresponding reduction in the heat transfer coefficient, which can increase the TWT further. As a result, heat transfer units for fired heater applications typically require multiple heating coils with a long length providing a sufficiently large surface area requiring substantial amounts of expensive materials (especially in high temperature circumstances) resulting in a process heater with a significant capital cost.
In conventional, tubular exchanger systems, heat transfer to the fluid may be improved using internal ribs, ridges, and/or grooves on the internal surfaces of tubes in order to increase the inner tube surface area. This provides increased heat transfer by conduction through the ribs, ridges, and/or grooves into the fluid core in addition to the heat transfer by convection via the flowing fluid. In systems using such ribs, ridges, and/or grooves, the flow regime in such systems typically are operated under conditions having a Reynolds Number (RE) less than about 250,000 and in some cases, much less than about 100,000. In such low RE flow regimes, applying internal ribs, ridges, and/or grooves can provide advantages to heat transfer, and generally the pressure drop through the tube due to the ribs, ridges, and/or grooves is not a significant concern due to the low velocities and relatively low levels of turbulence in the flow.
On the other hand, applying prior rib, ridge, and/or groove configurations to heat transfer units configured to operate at flow regimes having a high RE of about 250,000 or higher, such as a fired heater for catalytic naphtha reforming, generally results in a tube configuration with little or no advantage in heat transfer over a smooth walled tube equivalent. Such designs typically produce a significant pressure drop penalty associated with the ribbing, ridging, and/or grooving inside the tube due, among other factors, to the increased frictional contact between the fluid flow and the increased surface of the tube. In general, as the RE increases, the ratio of axial heat transfer from convection between ribs, ridges, and/or grooves relative to the radial heat transfer from conduction through the ribs, ridges, and/or grooves also increases. Such correlation indicates, as the RE increases in such systems, that there is a decreasing ability of the ribbing, ridging, and/or grooving to provide a benefit to the overall heat transfer via conduction relative to the heat transfer via convection. Accordingly, at such high RE ranges, it is generally understood that an internally ridged tube tends to exhibit the heat transfer performance of a bare tube equivalent (i.e., little improvement in heat transfer), and, at the same, time results in an undesirable increased pressure drop penalty. Thus, the heater tubes in a heat transfer unit operating at RE of about 250,000 or greater commonly do not include internal ribbing, ridging, and/or grooving because such structures provide little or no benefit to the heat transfer properties of the system and they produce pressure drop penalties and added capital cost.
Several studies have been reported of internal-axially-ridged tubes with empirical correlations for heat transfer and friction factor between the tube surface and the fluid flow. In one study, Kim and Webb suggested that an empirical correlation by Carnavos is the best available correlation for turbulent flow in axial and helical internally ridged tubes based on reliable data for air, water and water-glycol mixtures. (N. Kim and R. L. Webb, “Analytic Prediction of the Friction and Heat Transfer for Turbulent Flow in Axial Internal Fin Tubes,” Journal of Heat Transfer, Vol. 115 (1993), pp. 553-559 and T. C. Carnavos, “Heat Transfer Performance of Internally Finned Tubes in Turbulent Flow,” Heat Transfer Engineering, Vol. 1 (1980), pp. 32-37.) However, the range of Carnovos' data was limited to Prandlt Number (PR) of about 0.7≦Pr≦30 and Reynolds Number (RE) of about 10,000≦Re≦100,000. Such studies, therefore, provide little guidance for designing fired heaters in a RE flow regime of about 250,000 or greater.
In a fired heater configured for high temperatures and high RE, such as a fired heater for catalytic naphtha reforming, the heater configuration generally cannot be determined by only considering ridge configuration and process flow conditions. In many instances, configurations that may result in efficient heat transfer, low pressure drop, and satisfactory tube wall temperatures will also require long tube lengths or other designs that result in substantial amounts of expensive, exotic metals to construct suitable heaters for industrial use. As a result, a heater design that generally cannot provide improved heat transfer and decreased pressure drop for less material than an equivalent non-ridged heater is not desirable from a design or manufacturing standpoint. Such considerations are generally in contrast to the design of traditional steam boilers or heat exchangers at low RE and using more common materials, which do not result in a substantially negative effect to increased length and/or mass of the heater tubes.